The goal of this research is to elucidate the molecular and cellular mechanisms by which erbB receptors and their ligands influence mammalian development. Four specific aims are proposed: (1) To determine cellular mechanisms underlying the phenotypes in erbB2-deficient embryos; (2) To examine spatial and temporal expression of neuregulins and their receptors; (3) To determine whether erbB2 is essential for the development and function of other NC cell lineages and organ systems; and (4) To study signal transduction in erbB2-deficient embryos.